The present invention relates generally to reducing the acoustic noise caused by torque ripple in permanent magnet motors, and more particularly to reducing the acoustic noise without “tri-stating” the sinusoidal motor drive voltage that causes the torque ripple.
Permanent magnet motors generate a sinusoidal back electro-motive force (BEMF), and therefore require a sinusoidal drive signal to ensure low torque ripple so the acoustic noise generated by the motor will be low. The motor drive signal ordinarily is “tri-stated” for a time interval to synchronize the BEMF and a drive signal “profile” applied to drive the motor. This is necessary in order to measure the BEMF of the motor. The tri-stating of the motor drive voltages causes them to abruptly go to zero during the tri-stating interval, and this in turn causes the phase current to also go to zero during the tri-stating interval. The outputs of push-pull driver circuits (not shown) included in a motor driver circuit 5 (see subsequently described FIG. 2) are tri-stated, by turning off both the pull-up transistors and pull-down transistors thereof so that the drive voltage conductors are no longer being driven by the motor driver circuit. Since the output of each push-pull driver is tri-stated, it conducts a voltage that represents the motor BEMF. The segment A in subsequently described FIG. 1 indicates that the output of the push-pull driver is tri-stated, but the voltage generated by the motor on an output conductor of a push-pull driver actually is the motor BEMF.
The abrupt transition of the phase current to zero at the beginning of the tri-stating interval causes distortion in the motor phase current. (The term “tri-stating” of the motor drive voltage means that the pull-up transistors and pull-down transistors of the output stages of the motor driver circuitry are simultaneously turned off during the tri-stating interval, causing the voltages on the motor drive conductors to be determined only by the motor during the tri-stating interval.)
The motor torque is equal to the product of the phase current and the BEMF of the permanent magnet motor, so the above mentioned distortion in the phase current causes a substantial disturbance in the torque ripple, and therefore causes the motor to generate acoustic noise. The acoustic noise is dependent on the motor torque, and if the torque is constant, the acoustic noise is very low. However, if there is substantial ripple or variation in torque it results in a substantial amount of acoustic noise. (The acoustic noise is believed to be generated by small mechanical deformations caused by the torque ripple in material of which the motor is fabricated.)
Prior Art FIG. 1 shows waveforms for the sinusoidal drive voltage, the resulting phase current, and the sensed BEMF of a conventional permanent magnet motor. As indicated by segment “A” of the motor drive voltage waveform in FIG. 1, the drive voltage waveform is tri-stated for an interval during which a segment “B” of both the drive voltage waveform and the phase current waveform are zero. Therefore, the phase current is also considered to be tri-stated while phase current abruptly goes to zero and remains at zero during the tri-state interval B. After the tri-state interval B, the three waveforms in Prior Art FIG. 1 continue their normal sinusoidal variation for the negative portion of the sinusoidal cycle, wherein drive voltage, and hence the phase current, again are tri-stated as they approach the zero crossover level.
Prior Art FIG. 1 also shows the waveform of a sensed signal SYNC which represents the present motor speed. The BEMF voltage continues while a single drive voltage is being tri-stated by the motor driver circuit. The zero cross-over points of the BEMF voltage typically are sensed by means of an ordinary comparator in order to generate the BEMF zero crossover signal SYNC.
FIG. 1 also shows the torque characteristic of the motor. The normal torque level is indicated by “C”. The levels “D” of the torque characteristic indicate the large torque distortion or ripple caused by the phase current going to zero during the tri-state intervals B.
Other relevant prior art includes commonly assigned U.S. Pat. No. 6,252,362 entitled “Method and Apparatus for Synchronizing PWM Sinusoidal Drive to a DC Motor” issued Jun. 26, 2001 to White et al., incorporated herein by reference. This reference discloses a technique for tri-stating and synchronizing drive voltage to a DC permanent magnet motor to reduce acoustic noise of the motor. When the drive waveforms are properly synchronized, they cause a current in the motor windings that is in phase with the BEMF of the motor.
Microcontrollers, such as digital signal processors (DSPs), are commonly used to control the driver circuits that generate the drive voltages applied to permanent magnet motors. Techniques for using digital signal processors for sinusoidal driving of permanent magnet DC motors are disclosed in the technical article “Position Sensorless Brushless DC Motor/Generator Drives: Review and Future Trends” by T. Kim et al., IET Electr. Power Appl., Vol. 1, No. 4, July, 2007. Also see “Sensorless PM Brushless DC Motor Drives” by Nobuyuki Matsui, IEEE Transactions on Industrial Electronics, Vol. 43, No. 2, April 1996. Also see Texas Instruments Application Report SPRA588 entitled “Implementation of a Speed Field Oriented Control of 3-Phase PMSM Using TMS320” by Erwan Simon.
Unfortunately, the techniques including use of microcontrollers, DSPs, or the like to generate the drive voltages applied to permanent magnet motors are very complex, and sometimes include use of the well known Clarke-Park Transformation and PID (proportional-integral-derivative) controllers, and therefore are relatively expensive. The Clarke-Park Transformation is described in the Texas Instruments Application Report, Literature Number BPRA048, entitled Clarke & Park Transforms on the TMS320C2xx (1997). The Wikipedia article “PID Controller” cited in the Information Disclosure Statement submitted with the present application describes PID controller techniques.
Thus, there is an unmet need for a circuit and method for driving a permanent magnet electric motor so as to eliminate acoustic noise without tri-stating its drive voltages.
There also is an unmet need for a circuit and method for eliminating acoustic noise produced by an electric motor without tri-stating a motor drive voltage.
There also is an unmet need for a circuit and method for driving a permanent magnet electric motor without tri-stating the motor drive voltage wherein parameters of the motor are automatically determined.
There also is an unmet need for a circuit and method for driving a permanent magnet electric motor without the complexity and cost of utilizing a complex microcontroller such as a DSP.